Tetrahydrofuran is polymerized to manufacture polytetramethylene ether glycol, which is also referred to as poly(tetramethylene glycol) in the patent art. However, depending on source polytetramethylene ether glycol which is presently commercially available has low but significant levels of acidity which are practically irreducible from a commercial viewpoint. Acidity in the polytetramethylene ether glycol disturbs the stoichiometry of the isocyanate ingredient when the polytetramethylene ether glycol is used in the manufacture of polyurethane polymers, and strict stoichiometry is essential in the preparation of many thermo plastic polyurethane polymers. In addition, the presence of acidity may accelerate the rate of the isocyanate reaction leading to inconsistent and inconstant operating results when the polytetramethylene ether glycol varies from time to time in acid number. Also it is well appreciated that the presence of substantial acidity in the polytetramethylene ether glycol adversely affects some catalyst systems, also resulting in inconstant and inconsistent operations from day to day with respect to the manufacture of polyurethane polymers therefrom.
Thus, because of the relatively low levels of acidity, and because of the profound impact of acidity with respect to the operational characteristics of polytetramethylene ether glycol, commercially available polytetramethylene ether glycol has not been regarded as fungible goods. Thus, a large scale user of polytetramethylene ether glycol in the manufacture of polyurethane polymers is confronted with varying acid levels if multiple sources of the polymeg are to be relied on and this would result in erratic and inconsistent and inconstant operations and products. On the other hand, once a manufacturer of polyurethane polymers has polytetramethylene ether glycol of a particular acid number level in his storage facility it is understandable that he is reluctant to intermingle with this material polytetramethylene ether glycol of the same molecular weight from a different source and having a different acid number. This has resulted in a tendency on the part of purchasers of polytetramethylene ether glycol to cut out a source of polytetramethylene ether glycol other than the original source thus substantially reducing competition.
Polytetramethylene ether glycol is manufactured by the polymerization and tetrahydrofuran. U.S. Pat. No. 3,454,652 to Andrew P. Dunlop and Edward Sherman lists a large number of tetrahydrofuran polymerization catalysts. Tetrahydrofuran is polymerized by the admixing of a tetrahydrofuran polymerization catalyst with tetrahydrofuran and the resulting admixture is the so-called "living polymer" or catalytically active polymerization mixture. In accordance with the procedure of the aforementioned patent the equilibrium is shifted in favor of the polymer by lowering the temperature, but in any event the catalytic activity of the catalytically active tetrahydrofuran-catalyst mixture is terminated in due course by the addition of a catalytic activity terminating agent such as, for example, water or 1,4-butane diol. The resulting catalytically inactive admixture is then typically worked up by steaming, for example, or water washing, decanting to remove water, and treatment with organic solvents to further remove water. Typically the acidic residue remaining from the catalyst are neutralized by the addition of excess quantities of a solid calcium hydroxide, and in addition other solid adsorbents may be used to assist in the further purification of the polytetramethylene ether glycol. Thereafter the polytetramethylene ether glycol is filtered and subjected to vacuum distillation to remove organic solvents and traces of water. Nonetheless it has been found that regardless of the extent to which the polytetramethylene ether glycol is water-washed, or to which it is subjected to the solid alkaline neutralizing agents there appears to be a level of acidity which is reached and which has been apparently irreducible from a commercial point of view. These apparently irreducible levels may be quite low, for example, 0.01 to 0.05 mg. KOH/gram acid number, but nonetheless variations in this acid number causes the goods to be regarded as non-fungible commodities. Various systems and schemes have been suggested for reducing the acid number of the polytetramethylene ether glycol but it must be appreciated that mere reduction of the acid number to any level other than zero does not place the polytetramethylene ether glycol in the category of a fungible commodity because of the profound impact of acid number on the operating characteristics of the polytetramethylene ether glycol in polyurethane polymerizations.
It is an object of the present invention to provide a method of manufacturing polytetramethylene ether glycol in which the level of acidity is substantially zero thus providing the art, for the first time, with a means of placing polytetramethylene ether glycol in the category of fungible goods.